The invention is directed to a process and apparatus for the removal of nitrous gases from gaseous mixtures by the addition of oxidation agents and absorption of the nitric oxide in aqueous solutions. This process particularly serves for preliminary purification of gaseous mixtures which contain large amounts of nitrous gases in order to save expense in the later fine purification.
The absorption of nitrous gases is one of the most difficult problems to solve in the waste air purification art. One of the chief resistances hereby is in the poor solubility of nitrogen monoxide (NO) in aqueous systems. Among others, if the nitrous gases are present preponderantly as nitrogen dioxide (NO.sub.2), through disproportionation of the NO.sub.2 there is formed NO in the aqueous solution, which NO because of its poor solubility goes into the gas phase. This nitrogen monoxide then is only removed from the gas stream with great expense.
In the alkaline absorption of nitrogen oxides, theoretically there are formed nitrite and nitrate if at least equal parts by volume of NO.sub.2 and NO are present or the NO.sub.2 is in excess.
However, it has been shown in practice that in spite of excess liquor because of nonideal transportation of material in the liquid phase and the local over-acidification conditioned thereby that the reaction 3NO.sub.3 +H.sub.2 O.fwdarw.2HNO.sub.3 +NO occurs and NO is ascertained in the outlet of the absorber.
Besides large amounts of toxic materials are produced in the form of nirite and nitrate which are suited only in limited fashion for working up to fertilizer material.
It is known to remove nitrous gases from gas mixtures with reductive processes. For reducing absorption there are various processes. All they have in common is that there is employed a reducing material such as, e.g. sulfide or thiosulfate for the reduction of the NO.sub.2, whereby in the ideal case nitrogen is formed, but mostly dinitrogen monoxide (N.sub.2 O).
These reductive processes only function optimally if the nitrous gases are present exclusively as NO.sub.2. With an increasing portion of NO they are ineffective. Besides through the addition of the necessary reducing agent there arise high costs for chemicals. In most cases (e.g. with sulfur containing reducing agents) these reducing agents produce fresh toxic materials in the form of the oxidation product.
There are also known processes for the removal of nitrous gases from gaseous mixtures by oxidation of the nitric oxide. These are looked upon as ideal processes in which there is produced a further usable nitric acid from the nitric oxides without the formation of secondary toxic materials. Certainly there is needed a larger apparatus expense for the oxidation of the NO or the NO formed through the disproportionation of the NO formed in the gas phase.
The oxidation of concentrated nitrogen monoxide (NO) takes place spontaneously and exothermically with atmospheric oxygen to nitric oxide (NO.sub.2). Limited by the small probability of simultaneous successive meeting of one O.sub.2 and 2NO molecules in this gas phase, oxidation in this gas phase, oxidation with decreasing nitric oxide concentration, of course, there is always required longer residence times of the gaseous mixture in the reactor.
With the nitric oxide concentration occurring in the waste air sector this can lead to residence times of the order of magnitude of several minutes to hours. With the customary amounts of waste air it takes therethrough an oxidation space up to several hundred cubic meters.
In the case of the use of oxidation chemicals such as, e.g. hydrogen peroxide or ozone, there occur high costs for chemicals or high investment costs. To be sure here the requirement for producing no secondary toxic material is fulfilled in an ideal manner.
Another process which depends on the use of potassium permanganate (KMnO.sub.4) as the oxidation agent only functions if the nitric oxide consists of up to 50% NO.sub.2. The KMnO.sub.4 here merely has a catalytic function.
Theoretically the potassium permanganate is not consumed in this process but unavoidable losses of this expensive material in practice cause high costs. Besides thereby nitrite is produced which first must be worked up on else it causes waste water problems.
Furthermore, there are various processes for the gas reduction of nitrous gases, e.g. with hydrogen or even with ammonia, to nitrogen. Apart from the high consumption of hydrogen or ammonium these processes require temperatures above 250.degree. C. and the employment of catalysts. Disadvantageous hereby above all is the high energy expense and the sensitivity of the catalysts to impurities of the crude gas, as they are unavoidable, e.g. in the stainless steel pickling plants.
Therefore, it was the problem of the present invention to find a process and an apparatus for the removal of nitrous gases from gas mixtures by addition of an oxidation agent and absorption of the nitric oxides in aqueous solutions with which the nitrous gases can be removed quickly from the gaseous mixtures without formation of secondary waste and without high expense.